In this illustration, an asteroid is literally falling apart. This type of impactor can result in a chain of smaller craters rather than one large crater. Credit: NASA / JPL-Caltech.

Impact cratering is the excavation of a planet's surface when it is struck by a meteoroid. Impacts are instantaneous events. They leave very characteristic features.

Image from Mars Global Surveyor, courtesy of the Lunar and Planetary Institute.

What are craters?
Craters are roughly circular, excavated holes made by impact events. The circular shape is due to material flying out in all directions as a result of the explosion upon impact, not a result of the impactor having a circular shape (almost no impactors are spherical). Craters are the most common surface features on many solid planets and moons -- Mercury and our Moon are covered with craters.

What happens when an impactor hits?
When an impactor strikes the solid surface of a planet, a shock wave spreads out from the site of the impact. The shock wave fractures the rock and excavates a large cavity (much larger than the impactor). The impact sprays material -- ejecta -- out in all directions. The impactor is shattered into small pieces and may melt or vaporize. Sometimes the force of the impact is great enough to melt some of the local rock. If an impactor is large enough, some of the material pushed toward the edges of the crater will slump back toward the center and the rock beneath the crater will rebound, or push back up, creating a central peak in the crater. The edges of these larger craters also may slump, creating terraces that step down into the crater.

What are the major parts of a crater?

Floor -- The bottom of a crater, either bowl-shaped or flat, usually below the level of the surrounding ground.

Central peaks -- Peaks formed in the central area of the floor of a large crater. For larger craters (typically a few tens of kilometers in diameter) the excavated crater becomes so great that it collapses on itself. Collapse of the material back into the crater pushes up the mound that forms the central peak. At the same time, the rock beneath the crater rebounds, or bounces back up to add to the peak.

Walls -- The interior sides of a crater, usually steep. They may have giant stair-like terraces that are created by slumping of the walls due to gravity.

Rim -- The edge of the crater. It is elevated above the surrounding terrain because it is composed of material pushed up at the edge during excavation.

Ejecta -- Rock material thrown out of the crater area during an impact event. It is distributed outward from the crater's rim onto the planet's surface as debris. It can be loose materials or a blanket of debris surrounding the crater, thinning at the outermost regions.

Image from the Viking Orbiter, courtesy of the Lunar and Planetary Institute.

What are the different kinds of craters?

Simple craters are small bowl-shaped, smooth-walled craters (the maximum size limit depends on the planet).

The first small image shows a simple crater on Mars that has no central peak or terraces around its edges. The crater is 2 km (about 1 mile) wide. An extensive blanket of ejecta covers the area around the rim.

Complex craters are large craters with complicated features. Larger craters can have terraces, central peaks and multiple rings.

This next image shows a complex crater in the northern region of Mars. This crater is about 20 km (12 miles) across and has a large central peak and terraces around its rim. The ejecta blanket has lobes, which may indicate wet material was ejected, suggesting that subsurface water or melted ice was mixed into the debris.

Impact basins are very large impact structures that are more than 300 km (185 miles) in diameter. The largest impact basin on the Moon is 2,500 km (1,550 miles) in diameter and more than 12 km (7 miles) deep. Large impact basins are also found on other planets, including Mars and Mercury.

The large circular dark areas in the next image are impact basins, created as huge impactors struck the Moon. Lava later flowed across the low floors of the basins, giving them a darker, smoother appearance than the surrounding, brighter highlands. The dark basins can be seen by the naked eye.

Galileo Image (PIA00405), produced by the United States Geological Survey, courtesy of NASA.

Scientists describe other types of craters as well:

Multi-ring basins -- A very large impact basin surrounded by as many as five or six circular rings of mountain chains in addition to the main basin rim.

Irregular craters -- Craters with irregular shapes or multiple impact craters formed at the same time. Oblong craters can be created by impacts striking the surface at a very low angle.

Degraded craters -- Craters that have become eroded due to weathering, lava flows, impacting, or downslope movement of material.

How are large craters different than small ones?
Small craters often are simple bowl-shaped depressions. The structure of large craters is more complex because they collapse, forming terraces, central peaks, central pits, or multiple rings.

The largest impact craters are called impact basins.

What influences the size and shape of a crater?
The size and shape of the crater and the amount of material excavated depends on factors such as the velocity and mass of the impacting body and the geology of the surface. The faster the incoming impactor is, the larger the crater becomes. Typically, materials from space hit Earth at about 20 km (slightly more than 12 miles) per second. Such a high-speed impact produces a crater that is approximately 20 times larger in diameter than the impacting object. Smaller planets have less gravitational "pull" than large planets; impactors will strike at lower speeds. The greater the mass of the impactor, the greater the size of the crater.

Craters most often are circular. More elongate craters can be produced if an impactor strikes the surface at a very low angle -- less than 20 degrees.

How can craters be used to determine the age of a planet or moon?
Scientists record the size and number of impact craters -- and how eroded they are -- to determine the ages and histories of different planetary surfaces. Early in the formation of our solar system (before 3.9 billion years ago) there was lots of large debris striking the surfaces of the young planets and moons; these older impact basins are larger than the more recent craters. As a rule of thumb, older surfaces have been exposed to impacting bodies (meteoroids, asteroids and comets) for a longer period of time than younger surfaces. Therefore, older surfaces have more impact craters. Mercury and the Moon are covered with impact craters; their surfaces are very old. Venus has fewer craters; its surface has been covered recently (in the last 500 million years!) by lava flows that obscured the older craters. Much of Earth's surface is recycled through plate tectonic activity (and erosion), so Earth also has few craters.

Why does the Moon have so many craters while Earth has so few?
On Earth, impact craters are harder to recognize because of weathering and erosion of its surface. The Moon lacks water, an atmosphere and tectonic activity, three forces that erode Earth's surface and erase all but the most recent impacts. Approximately 80% of Earth's surface is less than 200 million years old, while over 99% of the Moon's surface is more than 3 billion years old. Essentially, the Moon's surface has not been modified since early in its history, so most of its craters are still visible.

What are some of Earth's famous impact craters?
Barringer Crater (Meteor Crater) in Arizona, United States, is a simple crater created when a 50-m-wide (160-foot-wide) iron-rich meteroid struck Earth's surface about 50,000 years ago -- a very recent event to a geologist. The crater is about 1.2 km (a little more than 0.5 miles) across and 200 m (650 feet) deep. Its features, such as the ejecta blanket beyond its rim, are well preserved because of the crater's youth; it has not experienced extensive erosion. Fragments of the Canyon Diablo meteorite were found inside the crater.

The Vredefort impact crater, about 100 km (60 miles) from Johannesburg, South Africa, was formed just a little over 2 billion years ago. It is the oldest and largest impact crater recognized on Earth's surface. The crater has been extensively eroded, but is believed to originally have been as much as 300 km (185 miles) across.

The Chicxulub crater in the Yucatan peninsula, Mexico, is not visible at the surface of the seafloor. Scientists rely on geophysical images for information about its size and shape. This crater is believed to have formed when an asteroid struck Earth 65 million years ago. This impact is thought to have triggered fires and tsunamis and created a cloud of dust and water vapor that enveloped the globe in a matter of days, resulting in fluctuating global climate changes. The extreme environmental shifts caused a mass extinction of 75% of Earth's species, including the dinosaurs.

How many objects from space impact Earth each year?
Earth and the other planets are constantly bombarded by tiny debris from space, much of which burns up in the atmosphere. Meteors -- incorrectly called shooting stars -- are the streaks of light created as particles of dust and ice vaporize in our atmosphere. Sometimes lots of particles strike at one time, creating meteor showers. Some of this tiny debris makes it to Earth's surface and is mixed with soil and ocean sediment.

Early in the formation of the solar system, frequent and large impacts were common for all of the planets and moons. This "period of heavy bombardment" ended by about 3.9 billion years ago. However, impacts still occur across our solar system, but at a reduced rate. Meteor Crater formed only 50,000 years ago. Earth continues to be a target -- and contrary to popular opinion, the Moon does not act as a meteoroid deflector (it is too small and too distant!). Scientists estimate that Earth and the other terrestrial planets are struck by, on average, five asteroids less than 2 km (a little over 1 mile) across every million years. Larger impacts also still occur, but these are much more rare.

Killer Asteroids
This site includes information on asteroid and comet impacts, and on the odds of Earth being struck in the future.

Origin of the Earth and Moon
The giant impact hypothesis suggests that the Moon formed as the result of a colossal impact onto the accumulating Earth, heating it and flinging the raw ingredients for the Moon into orbit around the Earth. The hypothesis is consistent with our ideas for how planets were assembled and explains some important features of the Earth-Moon system.